Silica-Supported Titania Species: Structural Analysis from Quantum

Article pubs.acs.org/JPCC

Silica-Supported Titania Species: Structural Analysis from Quantum Theory and X‑ray Spectroscopy C. S. Guo,† K. Hermann,*,† M. Hav̈ ecker,†,‡ A. Trunschke,† and R. Schlögl† †

Inorganic Chemistry Department, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Department Solar Energy Research, Helmholtz-Zentrum Berlin/BESSY II, Albert-Einstein-Strasse 15, 12489 Berlin, Germany

‡

ABSTRACT: Oxygen core excitations in different molecular titania−silica model clusters are evaluated using densityfunctional theory (DFT). These results are compared with in situ X-ray absorption fine structure (NEXAFS) measurements near the O K-edge of titania model catalysts supported on mesoporous silica, SBA-15, with low Ti loading. The comparison allows for an analysis of structural details of the supported titania species. The silica support is found to contribute to the NEXAFS spectrum in an energy range well above that of the titania units, allowing for a clear separation among the corresponding spectral contributions. The different bridging and terminal oxygen species can also be easily distinguished in the theoretical NEXAFS spectra. The experimental NEXAFS spectrum for 3 wt % Ti loading exhibits a single broad peak in the O 1s to the Ti 3d−O 2p excitation range of 530−534 eV. This can be explained by the theoretical data for tetrahedrally coordinated but not for triply coordinated TiOx species, suggesting the presence of OH groups at the expense of titanyl oxygen in the titania surface structure. The experimental NEXAFS spectrum for low Ti loading differs substantially from that of TiO2 anatase bulk with octahedral TiO6 units where the observed double-peak structure is also reproduced by the calculations.

1. INTRODUCTION Titanium dioxide has been extensively studied due to its interesting semiconducting and catalytic properties.1 The utilization of pure TiO2 as catalyst is centered on photochemical processes2 and a few other applications.3 Beyond that, titania has been used in particular as a carrier for noble metals4,5 and oxides, such as V2O5, MoO3, and WO3,6 due to its strong interaction with the supported active phase. This distinguishes TiO2 from silica which is regarded as a rather inert support. Titania supported vanadium oxide has attracted much attention as a catalyst for the selective oxidation of hydrocarbons or the selective catalytic reduction of nitric oxide.7,8 Recently, we have studied the properties of vanadia species supported on submonolayers and monolayers of titania hosted in the hierarchical pore structure of mesoporous silica, SBA-15, for the oxidative dehydrogenation of propane to propylene.9 The measurements for a broad range of titania and vanadia loadings showed that the surface topology of the vanadia species and its catalytic properties depend crucially on the titania loading where the catalysts have been prepared by the grafting of titanium and the sequential grafting of vanadium. At low titania loadings, the vanadia species is preferentially anchored at the previously deposited titania species forming a bilayer-supported catalyst. In contrast, at higher titania loadings, the vanadia species progressively bridge surface titanium oxide moieties forming Ti−O−V bonds in a common monolayer. The largest propylene productivity was observed when © 2012 American Chemical Society

vanadium oxide covers residual free patches of silica in submonolayer titania−silica supports, thus forming a combined V−Ti monolayer which masks the entire silica surface almost completely. The load-dependent behavior of surface titania in its reaction with vanadia is most likely associated with differences in the surface structure of the titanium dioxide species at low and high loadings. UV−vis and NEXAFS spectra at the Ti L-edge9 indicate progressive cross-linking with increasing titania loading. Therefore, information about the local environment of titanium atoms in submonolayer titania−silica supports is of particular importance for an understanding of the topology of the final vanadia catalyst and its catalytic properties. The molecular structure of titania incorporated in crystalline10,11 or amorphous12,13 silica has been studied by applying a broad spectrum of analytical techniques. X-ray absorption spectroscopy (XAS), in particular, at the Ti K-edge, has been used to examine the local structure of titanium in amorphous silica matrices. Titanium atoms turn out to be located predominantly in a tetrahedral environment when present in small amounts, while with increasing Ti content contributions of 6-fold coordinated Ti atoms become important.12 In particular, on a SBA-15 support, titania was found to yield different Received: August 13, 2012 Revised: October 1, 2012 Published: October 1, 2012 22449

dx.doi.org/10.1021/jp308047z | J. Phys. Chem. C 2012, 116, 22449−22457

The Journal of Physical Chemistry C

Article

O−Si bridges describing the titanium−substrate binding or in Ti−OH groups. Both Ti coordinations differ from those of bulk TiO2, anatase or rutile, where Ti is arranged in an (distorted) octahedral environment with six oxygen neighbors. The silica support in the titania−silica clusters is derived from polyhedral oligomeric silsesquioxane (POSS), Si8O12H8.16 This molecule is of cubic structure with silicon occupying the corners and oxygen near the edge midpoints, while hydrogen saturates the dangling bonds at the silicon corners (see Figure 1a). Titania species are added to this molecule, or to

coordinative behaviors. In the concentration range between 12 and 35 wt % TiO2, 4-fold-coordinated titanium was excluded on the basis of XAS measurements at the Ti K-edge. This suggests that for these titania concentrations, the TiO2 surface structure does not depend on the titania loading, and Ti atoms in octahedral and pentahedral coordination dominate.14 In contrast, for low Ti loadings on SBA-15, our recent in situ nearedge X-ray absorption fine structure (NEXAFS) measurements at the Ti L-edge9 indicate the existence of isolated TiO4 species. Here, the number of Ti−O−Ti bonds increases with the loading, but no TiO2 nanoparticles are formed up to a Ti loading of about 17 wt %.9 These findings are in accordance with the results of Raman and UV−vis spectroscopic analyses of these materials. Further, a strong dependence of the spectroscopic features on the titania loading is observed in NEXAFS measurements at the oxygen K-edge.9 Since the underlying structural changes are still unclear, we have performed density-functional theory (DFT) calculations on the electronic and geometric structure of titania−silica model clusters, simulating local sections of supported titania species. These studies are complemented by calculations on clusters representing sections of the anatase bulk TiO2 substrate, in order to identify characteristic differences that allow one to distinguish between the supported molecular species and titania nanoparticles. In addition to examining ground states and related properties of the clusters, we have evaluated core excitations of the different terminal and bridging oxygen species. Corresponding transition probabilities and energies are then used to determine theoretical excitation spectra which are compared with experimental in situ NEXAFS spectra at the oxygen K-edge of molecular TiOx species anchored at the surface of mesoporous silica, SBA-15. NEXAFS spectroscopy15 is based mainly on dipole excitations of highly localized core electrons by photons. Thus, corresponding measurements can provide reliable atom-specific information, allowing the differentiation of atoms in different local environments. This is used here to discriminate between differently bound oxygen atoms in terminal titanyl, TiO, as well as in Ti−O−Ti, Ti−O−Si, Ti−O−H, and Si−O−Si bridges of the silica-supported titania species. The comparison can provide a theoretical background that allows for a more detailed interpretation and assignment of the experimental spectra and contributes to the development of better catalysts. In Section 2, we introduce the models and discuss details of the computational methods used in the spectrum calculations. Section 3 describes experimental details connected with sample preparation and NEXAFS measurements, while Section 4 presents results and discussion. Finally, Section 5 summarizes our conclusions.

Figure 1. Geometric structure of the model clusters representing (a) the silica support (silsesquioxane, Si8O12H8) and (b) the anatase TiO2 bulk (Ti9O38H40) (see text). Atoms are shown by balls of different radii and labeled accordingly. The two nonequivalent oxygen species in the TiO2 bulk, O(a) and O(b), are indicated.

corresponding fragments, to yield titania−silica model clusters which are further optimized with respect to their geometry at the DFT level, including all atoms of the cluster (see below). The corresponding equilibrium geometries are shown in Figure 2. Here Figure 2, panels a−e represent clusters with tetrahedrally coordinated titanium: (a) Ti(OH)3−Si8O13H7 with the Ti(OH)3 group bound by one Ti−O−Si bridge to the silica support, (b) Ti(OH)2−Si7O12H8 with two Ti−O−Si bridges, (c) TiOH−Si7O12H7 with three Ti−O−Si bridges, (d) (TiOH)2−Si6O12H6 in which two Ti−O−H groups are connected by a Ti−O−Ti bridge, and each group binds with two Ti−O−Si bridges with the support, and (e) (TiOH)4− Si4O12H4 in which four polymeric Ti−O−H groups couple with the support by six Ti−O−Si bridges. Figure 2, panels f−i show equilibrium geometries of clusters where titanium is triply coordinated to oxygen with one titanyl double bond: (f) TiO-Si7O12H8 with one TiO and two Ti− O−Si bridges to the silica support, (g) (TiO)2−Si8O15H6 with two TiO bonds, one Ti−O−Ti bridge, and two Ti−O−Si bridges, (h) (TiO)2−Ti2Si6O15H6 with a titania network including two TiO bonds, four Ti−O−Ti bridges, and four Ti−O−Si bridges describing the coupling with the support, (i) TiO(OH)−Si8O13H7 containing both a TiO double bond and a Ti−O−H bridge with one Ti−O−Si bridge. Reference calculations on local titania sections in anatase TiO2 bulk are based on a Ti9O38H40 cluster (see Figure 1b), where the Ti9O38 part reflects the crystal structure of the ideal bulk. Dangling Ti−O bonds at the cluster periphery are saturated by hydrogen to account for cluster embedding. The Ti atoms in anatase TiO2 bulk are octahedrally coordinated with respect to their oxygen environment, which includes two nonequivalent oxygen species, O(a) and O(b), as indicated in Figure 1b. The different oxygen types have to be taken into account in corresponding core excitation and ionization calculations discussed below.

2. THEORETICAL DETAILS 2.1. Cluster Models. The present theoretical studies are based on titania−silica model clusters whose initial structures are obtained from chemical reasoning. The clusters represent a variety of arrangements of titania in different coordinations where the TiOx units are bound to silicon of the silica support via Ti−O−Si bridges. According to the oxidation state (4+) of titanium in a TiOx environment on silica, the metal species can be in a tetrahedral coordination with four Ti−O single bonds of which one, two, or three Ti−O−Si bridges provide the titanium−silica binding, while the others may be Ti−OH bonds. Alternatively, TiOx can be triply coordinated with one TiO titanyl double bond and two Ti−O single bonds in Ti− 22450



Article

Figure 2. Geometric structure of the titania−silica model clusters. The ball-and-stick models show clusters for (a−e) Ti in tetrahedral coordination with Ti−O−Si and Ti−O−H bond bridges, for (f−i) Ti triply coordinated with Ti−O−Si bridges and titanyl TiO groups. Atoms are shown by balls of different radii and labeled accordingly in (e) and (i).

2.2. Electronic Structure and NEXAFS Spectrum Calculations. The titania−silica clusters described above are geometrically optimized in electronic ground-state calculations, and corresponding equilibrium ground as well as oxygen core excited and ionized states are determined within densityfunctional theory (DFT) using the cluster code StoBe.17 Here Kohn−Sham orbitals are represented by extended basis sets of contracted Gaussians where vanadium, silicon, and hydrogen are represented by all-electron double-ζ-valence-plus-polarization (DZVP) basis sets. In the evaluation of X-ray absorption spectra, different basis sets are applied for oxygen. At the core, excited oxygen center IGLO-III type ([7s6p2d]) basis sets18 are used in order to adequately describe inner shell relaxation effects, while the other oxygen centers in the cluster are accounted for by effective core potentials (ECPs) for the 1s shell and [3s3p1d] valence basis sets to avoid 1s core orbital mixing,19,20 (see also reference 21). In the calculations, the gradient-corrected, revised Perdew-Burke-Ernzerhof (RPBE) exchange-correlation functional22,23 is employed. The computation of theoretical O 1s X-ray absorption spectra of the different clusters considers core to unoccupied orbital excitations resulting from dipole transitions. Thus, polarization-resolved spectral intensities I(E, e̲) are determined by the corresponding dipole transition matrix elements and vectors m̲ = (mx, my, mz), together with angle-dependent factors of the incoming radiation, characterized by the polarization vector e̲ = (ex, ey, ez), as ̲ ̲ 2, I(E , e)̲ = κE(me)

m̲ =

I (E ) =

∫ I(E , e)̲ dΩ = (2π /3)·κ·E·(mx 2 + my2 + mz 2) (2)

The evaluation of all core excited final states with corresponding transition energies, E, and matrix elements, m̲ , is achieved within the transition potential approach,24 in combination with a double basis set technique.25 This approximation is based on the use of a half occupied 1s core orbital at the corresponding oxygen excitation site in each cluster, thereby accounting for partial electronic relaxation due to the presence of the excited electron. It has been proven earlier that, within this approach, final-state relaxation effects are taken care of up to the second order in the energy, thus achieving a balance between the two relaxation mechanisms, core, and valence type.26 The transition energies and corresponding dipole transition matrix elements in eq 2 are convoluted using Gaussian broadening of varying width to simulate instrumental, vibrational, and lifetime broadening. A full-width-at-half-maximum (fwhm) value of 1 eV is applied below the ionization threshold, while the broadening is increased linearly to 4 eV up to 20 eV above threshold and kept fixed at this value for higher energies. In the transition potential approach, the electronic core hole relaxation of the excited final state is not fully accounted for. This incomplete relaxation can be corrected in an approximate way by shifting all excitation energies by the difference of the ionization potential computed with the transition potential method and the corresponding value from the ΔKohn−Sham (ΔSCF) calculations. This results in a global downward shift of about 2 eV. Further, relativistic corrections are included by applying an additional upward shift of the computed spectra by 0.73 eV.27 For further details of the method, consult references 24, 25, and 28−30. Successful applications to gas phase, adsorbed molecules, and surfaces can be found in references 29−37.

(1)

where κ is a global scaling factor; E denotes the transition energy, and the transition dipole vectors m̲ involve the initial core orbital, φcore, and final excited state orbitals, φf. In the present case where the titania can be assumed to bind at randomly oriented sites of the silica support, the spectral intensities, I(E, e̲), of eq 1 need to be averaged over all polarization directions, e̲, in order to compare with the experiment. This yields the polarization-averaged intensity as 22451



Article

calibration has been achieved by setting the π* resonance of the O2 gas phase signal at 530.9 eV, and the spectral resolution was about 150 meV. Further details of the methodology and data treatment are described elsewhere.40 Reference NEXAFS spectra of anatase TiO2 have been obtained at the O K-edge from pressed powder in the total electron yield (TEY) mode.

3. EXPERIMENTAL DETAILS 3.1. Synthesis of the Silica-Supported Titania. The silica-supported titania materials were prepared by grafting titanium(IV) isopropoxide [(Ti(OCH(CH3)2)4, Acros Organics 98+%] on the surface of mesoporous silica, SBA-15. The details have been described elsewhere.9 Briefly, SBA-15 was synthesized by up-scaling the method described in the literature.38 The alkoxide was then allowed to react with the dried silica surface by transferring an appropriate amount of an isopropanol solution that contains 25 wt % titanium(IV) isopropoxide to a SBA-15 suspension in isopropanol, stirring for 2 h at room temperature, filtering, and washing. The white filter cake was dried at 353 K in an 80 mbar dynamic vacuum for 2 h. Organic residues were removed by calcination in static air at 823 K for 2 h. The material that contains 3 wt % Ti (catalyst ID 7569) was prepared by applying one grafting step. The maximum loading that can be achieved in one grafting step avoiding undesired segregation of titania is approximately 8 wt % Ti. For titanium loadings higher than 8 wt %, the grafting procedure was repeated in a sequential manner with intermediate calcination. For the synthesis of 13 wt % Ti on SBA-15 (catalyst ID 9240), three grafting steps have been carried out. The silica materials loaded with a high amount of titanium (23 wt %) were prepared applying 9 steps (α, catalyst ID 11886) and 7 steps (β, catalyst ID 10380), respectively. The reference titanium oxide (anatase) was purchased from Alfa Aesar (99.9%). 3.2. NEXAFS Measurements. In situ near edge X-ray absorption fine structure (NEXAFS) measurements have been performed at the synchrotron radiation facility BESSY II of the Helmholtz-Zentrum Berlin, Germany (HZB) using monochromatic radiation of the ISISS (Innovative Station for In Situ Spectroscopy) beamline as a tunable X-ray source. Highpressure soft X-ray absorption spectra were measured in the presence of oxygen using the high pressure station designed and constructed at the Fritz Haber Institute, Berlin. Details of the setup are described elsewhere.39 The catalyst powders have been pressed into self-supporting discs (1t, 8 mm diameter) and mounted inside a cell onto a sapphire sample holder approximately 1.4 mm in front of the first aperture of a differentially pumped electrostatic lens system. The home-built electron lens serves as the input system for a modified commercial hemispherical electron analyzer (PHOIBOS 150, Specs-GmbH). Oxygen is introduced to the cell via a calibrated mass flow controller; heating is provided by a NIR laser at the rear of the sample, and the temperature is monitored by a thermocouple attached directly to the sample surface. NEXAFS spectra of the dehydrated samples have been obtained in 50 Pa O2 at 693 K by heating the material in situ in the XAS cell at 5 K/min up to the final temperature. Oxygen K-edge excitation spectra have been recorded in the Auger electron-yield mode by operating the electron spectrometer with a pass energy of 100 eV as an X-ray absorption spectroscopy (XAS) detector to minimize contributions from the gas phase to the spectra. The O K-edge spectra of the sample surface have been corrected for the remaining effects of the O2 gas-phase absorption. In order to increase the signal-to-noise ratio, data reduction by a factor of 2 has been applied to the raw spectra (containing about 1000 points per scan) by averaging adjacent points. Three scans have been averaged, and the X-ray spot position on the sample has been changed after each scan to avoid damage to the surface by the brilliant synchrotron X-ray beam. Absolute energy

4. RESULTS AND DISCUSSION Due to the well-defined preparation procedure, the present silica-supported titanium oxide/SBA-15 catalyst seems adequately suited as a model to gain new insight into the structure sensitivity of the NEXAFS spectra at the oxygen Kedge of surface metal oxides. The structure of the Ti/SBA-15 materials investigated in the present work has been studied earlier, applying complementary methods including thermal analysis, infrared spectroscopy, Raman, UV−vis spectroscopy, electron microscopy, and NEXAFS at the Ti L2,3-edge.9 These measurements confirm that the titanium oxide species were grafted on the walls of the meso- and micropores of SBA-15 in sub-monolayer quantities as highly dispersed, two-dimensional (2D) titania surface species without noticeable segregation of titanium oxide nanoparticles for 3 and 13 wt % Ti/SBA-15. Raman spectroscopy shows a transition from well-defined, isolated titanium oxide species at 3 wt % Ti/SBA-15 to twoand three-dimensional (3D) precursors of the solid state at 23 wt % Ti/SBA-15.9 Further, the results from X-ray diffraction (XRD) exclude the presence of larger crystalline particles in 23 wt % Ti/SBA-15 α but indicate the presence of titania nanoparticles in 23 wt % Ti/SBA-15 β. 4.1. NEXAFS Spectra for Anatase Bulk TiO2. Before examining the silica-supported TixOy species, we validate our theoretical methods for anatase bulk TiO2, whose crystal structure is well-known.41 For this compound polarizationaveraged oxygen K-shell, NEXAFS spectra have been measured in the present study using polycrystalline powder material. Figure 3 compares the calculated oxygen 1s core excitation

Figure 3. Comparison of the theoretical oxygen 1s core excitation spectrum (theor.) evaluated for the model cluster, Ti9O38H40 (see Figure 1), with an experimental O K-edge NEXAFS spectrum for anatase TiO2 bulk (exp.) in the energy range between 528 and 538 eV. The two vertical dashed lines indicate the energy range of the computed ionization potentials of the two nonequivalent bulk oxygen species, O(a) and O(b), shown in Figure 1.

spectrum for a model cluster Ti9O38H40, representing a crystal section of anatase TiO2 bulk, with corresponding experimental O K-edge NEXAFS data for samples of anatase bulk. (The model cluster, shown in Figure 1b, is terminated by hydrogen at its periphery to simulate bulk embedding.) Both spectra reveal a double-peak structure for excitation energies near 531.0 and 533.5 eV where the comparison proves very good agreement, considering the fact that an absolute energy scale without adjustment between theory and experiment is used. This gives 22452



Article

loading can also result in a double-peak spectrum β (see Figure 4) with peaks at 530.9 eV (0.9 eV fwhm) and 533.3 eV (1.0 eV fwhm). This spectrum is reminiscent of the double-peak structure with peaks at 531.0 and 533.5 eV, which is observed for crystalline anatase TiO2 bulk and is also included in Figure 4. The similarity of spectrum β may indicate nanosized TiO2 crystallites, which is also evident from the XRD results. Corresponding diffraction patterns are in agreement with the titania modification anatase. Crytallite sizes [LVol-IB (i.e., volume-weighted mean column length based on integral width)] of 4.5 ± 0.5 nm can be estimated when only crystal size effects are considered in the fit using the program TOPAS (Bruker AXS). If, in addition, strain is allowed to contribute to peak broadening, the calculated crystallite size is estimated as 5.0 ± 0.8 nm. Thus, anatase nanoparticles of a crystallite size of approximately 5 nm are found to contribute to spectrum β. On the other hand, the NEXAFS spectra for low titania loading, such as 3 wt %, look rather different from the bulk spectrum. This suggests a different geometric arrangement of the oxygen in deposited titania compared with that inside anatase TiO2 bulk, in which distorted octahedral TiO6 units exist. Higher loadings of titania deposit, 13 and 23 wt % (α spectrum), which broaden and shift the single NEXAFS peak, may result in additional structural features to be explained. 4.3. Theoretical NEXAFS Spectra for TixOy−SiO2 Model Clusters. 4.3.1. Model Clusters with Ti−O Single Bonds Only. Figure 5 shows theoretical O 1s core excitation spectra for the

confidence that the present theoretical methods can also yield a reliable description of the spectroscopic data for small supported TixOy particles. The theoretical bulk spectrum of Figure 3 can be characterized in detail by analyses of the corresponding finalstate orbitals. As a first result, the O 1s core excitations of the two nonequivalent oxygen species in the bulk, O(a) and O(b) (see Figure 1b), yield basically identical spectral features. (The theoretical spectrum in Figure 3 refers to O(a) core excitations.) Thus, the discussion can be restricted to one of the two [e.g., O(a)]. The two theoretical peaks shown in Figure 3 are found to be dominated by O 1s core excitations to unoccupied orbitals with antibonding O 2p−Ti 3d character. Here, the peak at lower energy, 531.0 eV, refers to final-state orbitals where the O 2p admixture points perpendicular to the corresponding O−Ti bond, whereas the peak at higher energy, 533.5 eV, originates from excitations to orbitals with the O 2p admixture pointing along the O−Ti bond. Thus, the different amount of antibonding character of the two types of orbitals explains the energetic peak sequence. This result is qualitatively analogous to those found earlier for vanadia21 and molybdena particles42 on silica support. 4.2. Experimental NEXAFS Spectra for Supported TixOy Species on SBA-15. Figure 4 compares experimental

Figure 4. Experimental O K-edge NEXAFS spectra for dehydrated TixOy/SBA-15 with 3 to 23 wt % Ti loading (see text). The NEXAFS spectrum for anatase TiO2 bulk from Figure 3 is included for comparison. Figure 5. Theoretical O 1s core excitation spectra for clusters (a)−(e) containing tetrahedrally coordinated titanium forming four Ti−O single bonds (see Figure 2). The spectra are compared with the experimental O K-edge NEXAFS data for dehydrated TixOy/SBA-15 with 3 wt % Ti taken from Figure 3.

oxygen K-edge NEXAFS spectra of adsorbed TixOy for three different titanium loadings, 3 wt %, 13 wt %, and 23 wt %, at the SBA-15 support where the measurements refer to an excitation energy range between 526 and 538 eV. (The figure also includes the anatase bulk spectrum of Figure 3.) For a 23 wt % titanium loading, two different spectra, α and β, are obtained depending on the sample preparation (see Synthesis of the Silica-Supported Titania and below. The figure does not include the oxygen K-edge NEXAFS spectrum for the SBA-15 support itself which was measured earlier42 and yields no excitations below 535 eV, in agreement with theoretical studies.42 The spectrum of dehydrated titania at the support shows for 3 wt % loading, one broad peak at 532.3 eV with 1.3 eV fwhm originating from core excitations of oxygen located in the TixOy species. This peak is broadened further and shifts to slightly lower energies when the titania loading is increased, for 13 wt % to 531.9 eV with 2.6 eV fwhm and for 23 wt % (α spectrum) to 531.7 eV with 3.2 eV fwhm. With a different preparation technique, applying only 7 instead of 9 grafting steps, 23 wt %

five titania−silica model clusters, (a)−(e) of Figure 2, where the titanium of the titania subunits is tetrahedrally coordinated to oxygen, forming four Ti−O single bonds. Here the oxygen is bridging between titanium in TixOy oligomers, between titanium and silicon determining the binding of the TixOy species with the silica support, as well as between titanium and hydrogen originating from the OH groups at the titanium atom. Therefore, the theoretical total spectra of Figure 5 are obtained from stoichiometrically weighted superpositions of the partial spectra due to the core excitation of the different types of oxygen. This yields, in all cases, a broad single peak between 530 and 534 eV with only slightly varying asymmetries. The spectral similarity suggests that the actual geometric coupling of the titania species with the silica support, by one Ti−O−Si bridge in cluster (a) of Figure 2 and by two bridges in clusters 22453



Article

(b)−(e), influences the spectral features only slightly. Likewise, the similarity of the spectra for clusters (c), (d), and (e), in which one to three (adjacent) OH groups binding with titanium in addition to Ti−O−Ti bridges exist, seems to also indicate that these structural properties do not affect the spectrum noticeably. A detailed analysis of the partial excitation spectra for the different oxygen species in cluster (c) [see Figure 6 (top)]

Figure 7. Theoretical O 1s excitation spectra calculated for oxygen in Ti−O−H (top) and Ti−O−Ti bridges (bottom) of cluster (d) (see Figure 2). The figure includes discrete excitation energies given by vertical lines of lengths characterizing corresponding excitation probabilities. The iso-surface plots illustrate representative final-state orbitals (1), (2) near the centers of the two peaks of Ti−O−H and for the two excitations with the largest probabilities of Ti−O−Ti (see text).

Figure 6. Decomposition of the core excitation spectra into contributions from different oxygen species in clusters (c) and (d) (see Figure 2). Oxygen in Ti−O−H, Ti−O−Si, and Si−O−Si bridges.

determines the energetic order of the excitations. As an illustration, Figure 7 (top) shows iso-surface plots of two finalstate orbitals resulting from (1) the topmost excitation of the lowest peak and (2) the main excitation of the higher peak. These orbitals are focused mainly in the Ti−O−H region of model cluster (d) with only very little orbital admixture from more remote oxygen and silicon atoms. The orbital shapes prove 3d type character near the titanium center and 2p character near the oxygen excitation center, labeled “O” in the right plot. For final-state orbital (1), the corresponding O 2p function points perpendicularly to the Ti−O bond axis, yielding less antibonding character than found for orbital (2), where the p function points almost along the Ti−O bond axis. The lower part of Figure 7 shows for oxygen core excitations at the Ti−O−Ti bridge of model cluster (d), an asymmetric single peak centered at 532.1 eV with a small low-energy shoulder at 531.2 eV. This originates from eight discrete excitations where excitation (1) at 531.2 eV in Figure 7 (bottom) determines the peak shoulder while excitation (2) at 532.5 eV dominates in intensity at higher energy. The isosurface plots of the corresponding final-state orbitals given in Figure 7 (bottom) are qualitatively analogous to those shown for excitations in the Ti−O−H region. They are both focused mainly at the Ti−O−Ti region and described as antibonding combinations of Ti 3d and O 2p functions where the amount of antibonding character determines their energetic order. Figure 5 also includes the experimental O K-edge NEXAFS spectrum obtained for dehydrated TixOy species at the SBA-15 support with low Ti loading of 3 wt % (see also Figure 4). This spectrum resembles, in the energy region between 530 and 534 eV, the theoretical spectra for the model clusters (a)−(e) quite closely. Here it should be noted that the energy scale describing the positions of the single-peak structures has not been adjusted between theory and experiment. This indicates that dehydrated TixOy supported on SBA-15 can be characterized structurally as tetrahedrally coordinated Ti where binding to the support is achieved by Ti−O−Si bridges, while the other oxygen species not appearing in Ti−O−Ti bridges is saturated by hydrogen,

shows that the broad peak between 530 and 534 eV is dominated by oxygen excitations in Ti−O−H and Ti−O−Si bridges. When additional Ti−O−Ti bridges appear in larger TixOy units, such as those simulated by clusters (d) and (e), corresponding oxygen core excitations contribute intensity in the same energy range. This is obvious from Figure 6 (bottom), which shows the spectral decomposition for cluster (d). In contrast, contributions from oxygen in Si−O−Si bridges of the silica support appear in all clusters only above 535 eV. The qualitative peak characterization is found to be very similar in all clusters (a)−(e) despite their different distribution of Ti−O−H, Ti−O−Ti, and Ti−O−Si bridges. This suggests, in particular, that the O 1s excitation in Ti−O−Ti bridges cannot be distinguished from that in Ti−O−H and Ti−O−Si bridges. It may further suggest that, for higher titania loading, the single peak structure will not be greatly affected beyond a broadening due to the cluster size-dependent hybridization of corresponding cluster final-state orbitals. This can explain the present experimental results for the loading dependence of the O K-edge NEXAFS spectra between 3 and 23 wt %. Further insight into the type of oxygen core excitations determining the single-peak spectrum in the energy region between 530 and 534 eV can be gained by examining corresponding final-state orbitals. As an example, Figure 7 shows the partial spectra obtained for core excitations at the bridging oxygen in Ti−O−H (upper part) and Ti−O−Ti bridges (lower part) of cluster (d) (see Figure 2). This figure also includes discrete excitation energies given by vertical lines of lengths characterizing corresponding excitation probabilities. The upper part of Figure 7 shows for oxygen core excitations at the Ti−O−H bridges a multipeak spectrum where the energetically lowest peak at 532.2 eV originates from five discrete excitations. The second-lowest peak near 534.2 eV combines two excitations separated by 0.2 eV where the higher excitation, denoted (2) in the following, contributes more intensity. In all cases, the core excitations are characterized by final-state orbitals described as antibonding mixtures of Ti 3d and O 2p functions, where the amount of antibonding character 22454



Article

found for clusters (g) and (h) which contain two and four triply coordinated Ti species with titanyl oxygen. These clusters include, in addition, oxygen in Ti−O−Ti bridges with corresponding O 1s excitations described by multiple peaks, which overlap in part with the titanyl oxygen peaks and extend to higher energies. Model cluster (i) includes triply coordinated titanium with both titanyl TiO and Ti−O−H bridges. In addition, the TiO3 unit is bound to the support by only one Ti−O−Si bridge. Here, the O 1s spectrum of the Ti−O−H bridges forms a two-peak structure located at quite similar energies as found for clusters (a)−(e), containing only OH groups without titanyl oxygen. In contrast, the titanyl oxygen core spectrum for cluster (i) is shifted by 2−3 eV with respect to those of the other titanyl-containing clusters (f)−(h). This may be explained by the local charging effects due to the presence of the OH group but remains to be clarified in detail. Altogether, the theoretical O 1s core excitation spectra for clusters (f)−(i), which contain titanyl oxygen in TiO double bonds, show rather complicated multipeak structures depending on cluster size and shape. This is quite different from the results for clusters (a)−(e) where titanyl oxygen does not appear to lead to broad single-peak spectra. The latter is consistent with the experimental O K-edge NEXAFS data for dehydrated TiOx/SBA-15 at 3 wt % Ti loading (gray spectrum exp. in Figure 8, bottom panel) which was shown to also yield a single-peak structure between 530 and 534 eV. Thus, the comparison between theory and experiment strongly suggests that the measured NEXAFS spectrum for low Ti loading cannot be explained by model results for triply coordinated TixOy species including titanyl oxygen, in agreement with Raman spectroscopy.9

which results in OH groups and Ti−O−H bridges. In fact, two different Ti−OH species, characterized by bands at 3720 cm−1 and 3670 cm−1, have been observed by infrared spectroscopy.9 This is in agreement with the present computational results explicitly excluding titanyl oxygen with TiO double bonds. However, for a more conclusive structural confirmation, other titania types need to be considered and will be discussed below. It should be emphasized that, while the theoretical spectra for clusters (a)−(e) reproduce the experimental NEXAFS data for low Ti loading quite well, a further discrimination between the clusters based only on the agreement with experimental spectra seems questionable. A clear assignment of the most appropriate cluster describing the NEXAFS experiment cannot be made due to the approximate nature of the model clusters (a)− (e). 4.3.2. Model clusters with TiO Double Bonds. Figure 8 shows theoretical O 1s core excitation spectra for the four

Figure 8. (Left, “Total”) Theoretical O 1s core excitation spectra for clusters (f)−(i) containing triply coordinated titanium forming two Ti−O single and one TiO double bond (see Figure 2). The spectra are compared with experimental O K-edge NEXAFS data for dehydrated TixOy/SBA-15 with 3 wt % Ti taken from Figure 3. (Right, “Atom res.”) Decomposition of the excitation spectra into contributions from different oxygen species in clusters (f)−(i) (see Figure 2) , oxygen in titanyl TiO, and oxygen in Ti−O(*)−Ti, Ti− O−H, and Ti−O−Si bridges.

5. CONCLUSIONS In the present study, we have evaluated oxygen 1s core excitation spectra of clusters simulating local sections of anatase TiO2 bulk and of silica-supported titania model clusters using DFT cluster methods. The results are compared with spectra of in situ O K-edge NEXAFS measurements for polycrystalline anatase TiO2 bulk as well as for dehydrated titania of different concentration on SBA-15 support. This comparison helps to get further insight into possible structural features of the titania species examined in catalytic studies.9 In the calculations, the partial core excitation spectra of differently binding oxygen species, titanyl TiO, as well as oxygen in Ti−O−H, Ti−O−Ti, Ti−O−Si, and Si−O−Si bridges of the silica-supported titania species, are found to vary crucially in their energetic peak distributions and peak shapes. Therefore, their spectral superposition in different titania units, tetrahedrally coordinated Ti including OH groups or triply coordinated Ti with double-bonded titanyl oxygen, can be used to distinguish between the corresponding species in welldefined monolayer systems and to aid in conclusions about possible existence of different titania species. Experimental O K-edge NEXAFS data for samples of anatase TiO2 bulk exhibit a clean double-peak structure with a low energy peak near 530.9 eV and a high energy peak at 533.4 eV (see Figure 3). This spectrum is reproduced by the computed total spectrum of O 1s core excitations in the model clusters for anatase TiO2 bulk. On the other hand, O K-edge NEXAFS spectra of low Ti loadings (3−13 wt %) on the SBA-15 support for energies between 530 and 534 eV, yield a single-peak structure which differs substantially from that obtained for the

titania−silica model clusters, (f)−(i) of Figure 2, where the titanium of the titania subunits is triply coordinated to oxygen, forming two Ti−O single and one TiO double bond. Thus, the clusters differ from those, (a)−(e), discussed earlier by the appearance of double-bonded titanyl oxygen. As before, the theoretical total spectra, shown in the left diagram of Figure 8 (labeled “Total”), are calculated as stoichiometrically weighted superpositions of the partial spectra due to core excitation at the different types of oxygen, terminal titanyl and 2-fold bridging. The right diagram (labeled “Atom res.”) shows a decomposition of the theoretical excitation spectra into contributions from different oxygen species. Obviously, the total core excitation spectra of Figure 8 yield, for all clusters (f)−(i), multipeak structures which differ noticeably between different clusters. This is explained by the rather different structure of the TiO3 units and their coupling with the silica support in the clusters. In cluster (f), the triply coordinated Ti species connects with the silica support by two Ti−O−Si bridges. Here titanyl oxygen contributes the dominant intensity to the excitation spectrum in the energy range between 529 and 533 eV, resulting in two major peaks, while oxygen in Ti−O−Si bridges provides much less intensity, becoming important at higher energies. This behavior is also 22455



Article

(8) Párvulescu, V. I.; Grange, P.; Delmon, B. Catal. Today 1998, 46, 233−316. (9) Hamilton, N.; Wolfram, T.; Tzolova Müller, G.; Hävecker, M.; Kröhnert, J.; Carrero, C.; Schomäcker, R.; Trunschke, A.; Schlögl, R. Catal. Sci. Technol. 2012, 2, 1346−1359. (10) Vayssilov, G. N. Catal. Rev. 1997, 39, 209−251. (11) Arends, I. W. C. E.; Sheldon, R. A.; Wallau, M.; Schuchardt, U. Angew. Chem., Int. Ed. 1997, 36, 1144−1163. (12) Gao, X.; Wachs, I. E. Catal. Today 1999, 51, 233−254. (13) Davis, R. J.; Liu, Z. F. Chem. Mater. 1997, 9, 2311−2324. (14) Zhang, F.; Carrier, X.; Krafft, J.-M.; Yoshimura, Y.; Blanchard, J. New J. Chem. 2010, 34, 508−516. (15) Stöhr, J. In NEXAFS Spectroscopy; Gomer, R., Ed.; SpringerVerlag: Berlin, 1992. (16) Feher, F. J.; Walzer, J. F. Inorg. Chem. 1991, 30, 1689−1694. (17) Hermann, K.; Pettersson, L. G. M. StoBe software, version 3.1; deMon Developers Group, 2011 (http://www.fhi-berlin.mpg.de/ KHsoftware/StoBe/). (18) Kutzelnigg, W.; Fleischer, U.; Schindler, M. NMR Basic Principles and Progress; Springer-Verlag: Berlin, 1990. (19) Nyberg, M. Ph.D. Thesis, Stockholm University, 2000. (20) Pettersson, L. G. M.; Wahlgren, U.; Gropen, O. J. Chem. Phys. 1987, 86, 2176−2184. (21) Cavalleri, M.; Hermann, K.; Knop-Gericke, A.; Hävecker, M.; Herbert, R.; Hess, C.; Oestereich, A.; Döbler, J.; Schlögl, R. J. Catal. 2009, 262, 215−223. (22) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 7413−7421. (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Triguero, L.; Pettersson, L. G. M.; Ågren, H. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 8097−8110. (25) Ågren, H.; Carravetta, V.; Vahtras, O.; Pettersson, L. G. M. Chem. Phys. Lett. 1994, 222, 75−81. (26) Slater, J. C.; Johnson, K. H. Phys. Rev. B: Condens. Matter Mater. Phys. 1972, 5, 844−853. (27) Takahashi, O.; Pettersson, L. G. M. J. Chem. Phys. 2004, 121, 10339−10345. (28) Leetmaa, M.; Ljungberg, M. P.; Lyubartsev, A.; Nilsson, A.; Pettersson, L. G. M. J. Electron Spectrosc. Relat. Phenom. 2010, 177, 135−157. (29) Kolczewski, C.; Püttner, R.; Plashkevych, O.; Ågren, H.; Staemmler, V.; Martins, M.; Snell, G.; Schlachter, A. S.; Sant’Anna, M.; Kaindl, G.; et al. J. Chem. Phys. 2001, 115, 6426−6437. (30) Kolczewski, C.; Hermann, K. Surf. Sci. 2004, 552, 98−110. (31) Kolczewski, C.; Püttner, R.; Martins, M.; Schlachter, A. S.; Snell, G.; Sant’Anna, M.; Hermann, K.; Kaindl, G. J. Chem. Phys. 2006, 124, 034302−034314. (32) Cavalleri, M.; Näslund, L. Å.; Edwards, D. C.; Wernet, P.; Ogasawara, H.; Myneni, S.; Ö jamäe, L.; Odelius, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem. Phys. 2006, 124, 194508−194515. (33) Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Näslund, L. Å.; Hirsch, T. K.; Ojamae, L.; Glatzel, P.; et al. Science 2004, 304, 995−999. (34) Ö ström, H.; Ogasawara, H.; Näslund, L. Å.; Pettersson, L. G. M.; Nilsson, A. Phys. Rev. Lett. 2006, 96, 146104−146107. (35) Schiros, T.; Haq, S.; Ogasawara, H.; Takahashi, O.; Ö ström, H.; Andersson, K.; Pettersson, L. G. M.; Hodgson, A.; Nilsson, A. Chem. Phys. Lett. 2006, 429, 415−419. (36) Kolczewski, C.; Hermann, K. J. Chem. Phys. 2003, 118, 7599− 7609. (37) Cavalleri, M.; Hermann, K.; Guimond, S.; Romanyshyn, Y.; Kuhlenbeck, H.; Freund, H.-J. Catal. Today 2007, 124, 21−27. (38) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024−6036. (39) Knop-Gericke, A.; Kleimenov, E.; Hävecker, M.; Blume, R.; Teschner, D.; Zafeiratos, S.; Schlögl, R.; Bukhtiyarov, V. I.; Kaichev, V. V.; Prosvirin, I. P.; et al. X-Ray Photoelectron Spectroscopy for Investigation of Heterogeneous Catalytic Processes. In Advances in

anatase bulk. This suggests different local geometric structures of the corresponding TixOy species. Here, the present theoretical analysis of oxygen 1s core excitation spectra for different titania units in various titania−silica clusters can give a rather clear assignment. Oxygen core excitations from Ti−O− H, Ti−O−Si, and Ti−O−Ti bridges all contribute in the energy range between 530 and 534 eV. In contrast, spectral contributions from core excited oxygen in Si−O−Si bridges of the silica support are found to appear only above 535 eV (i.e., energetically well separated from the observed double-peak structure). Further, the model calculations for clusters, which include double-bonded titanyl oxygen, result in spectra with multipeak structure in the energy range between 530 and 534 eV, which is not observed in the experimental TixOy/SBA-15 NEXAFS spectra for low Ti loading. On the other hand, for clusters which do not include double-bonded titanyl oxygen (i.e., all Ti−O bonds are either components of Ti−O−Si bridges, characterizing titania-support binding, or appear as Ti− O−H bridges involving OH groups), the theoretical spectra can reproduce the experimental findings quite nicely. This suggests that, with the present sample preparation, the titania species at low loading on a SBA-15 support may be assumed to exclude titanyl oxygen and involves only OH groups bound to the central titanium atom. Theoretical and experimental studies of other titania reference compounds, such as rutile- and titanyl-containing compounds whose structures are well-known, may help to get deeper inside into the structure of the supported titania catalysts. Studies along these lines are currently under way. Finally, EXAFS analyses at the Ti K edge of the supported titania species may provide additional information about the number of oxygen atoms in the neighborhood of Ti centers. However, such studies require, apart from hard X-ray measurements, completely different theoretical tools which are beyond the scope of the present work.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Till Wolfram and Daniel Brennecke for synthesis of the silica-supported titania. Detre Teschner is acknowledged for providing the O K-NEXAFS spectrum of TiO2 anatase. We thank Frank Girgsdies for detailed XRD analyses. The Helmholtz-Zentrum Berlin (HZB) staff is acknowledged for continuously supporting the synchrotronbased high pressure electron spectroscopy experiments of the Fritz-Haber Institute at BESSY II.

■

REFERENCES

(1) Kamat, P. V. J. Phys. Chem. C 2012, 116, 11849−11851. (2) Anpo, M. Res. Chem. Intermed. 1989, 11, 67−106. (3) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 25, 61− 69. (4) Tauster, S. J. Acc. Chem. Res. 1987, 20, 389−394. (5) Haller, G. L.; Resasco, D. E. Metal−Support Interaction: Group VIII Metals and Reducible Oxides. In Advances in Catalysis; Eley, D. D., Eds.; Academic Press Inc.: San Diego, 1989; Vol. 36, pp 173−235. (6) Bond, G. C.; Tahir, S. F. Appl. Catal. 1991, 71, 1−31. (7) Grzybowska-Swierkosz, B. Appl. Catal., A 1997, 157, 263−310. 22456



Article

Catalysis; Gates, B. C., Knö zinger, H., Eds.; Academic Press: Amsterdam, 2009; Vol. 52, pp 213−272. (40) Hävecker, M.; Cavalleri, M.; Herbert, R.; Follath, R.; KnopGericke, A.; Hess, C.; Hermann, K.; Schlögl, R. Phys. Status Solidi B 2009, 246, 1459−1469. (41) Howard, C. J.; Sabine, T. M.; Dickson, F. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1991, 47, 462−468. (42) Guo, C. S.; Hermann, K.; Hävecker, M.; Thielemann, J. P.; Kube, P.; Gregoriades, L. J.; Trunschke, A.; Sauer, J.; Schlögl, R. J. Phys. Chem. C 2011, 115, 15449−15458.

22457


Silica-Supported Titania Species: Structural Analysis from Quantum

Recommend Documents